Environmental carcinogens are implicated in the majority of human cancers. Hence, there is a need for an in vitro assay for screening of potential chemical carcinogens. Cultured animal cells can be used for a direct study of malignant transformation. A rountine in vitro assay for carcinogenic potential in animal cells should employ a marker of oncogenic transformation that is reliable, easily performed, readily scored and inexpensive. Two potential markers are being characterized in this laboratory. Tumorigenic epithelial cells cultured in medium supplemented with cytochalasin B exhibit uncontrolled nuclear division which is readily quantitated by determining the number of nuclei/cell in a representative sample of cells; nontumorigenic cells are predominately mono- and binucleate. Tumorigenic cells also are altered in the relative labeling of two novel aminoacyl fucosides, FL3 and FL4. In addition, clonal epithelial cell lines derived from a mouse mammary carcinoma which are very weakly tumorigenic at low cell passage appear to be good candidates for tester cell lines in an in vitro assay of carcinogenicity. The relationship between tumorigenicity and the in vitro markers described above will be examined directly by animal testing. Cell lines examined will include those cited above as well as additional cultured epithelial cells and one fibroblastic cell line. Optimal conditions for assay of different classes of chemical carcinogens will be determined in order to develop an assay which is suitable for a broad range of environmental carcinogens.